Clean as well as you can, then lubricate. Repeat as necessary.
I have had successful results with WD-40 in several applications, followed by oil.
I also wouldn't hesitate too much to flush with some mild solvent then lubricating.
As Sheldon Brown points out on his site, you can take freewheels apart and do it right, ut it isn't really necessary since it is about the least critical of the bearings on a bike, only used when you are not pedalling.